The present invention relates to a power semiconductor device in which a drift region is constituted by a semiconductor material having a band gap wider than that of silicon (Si) (referred to hereinbelow as WBG semiconductor).
It has been reported (see, for example, Japanese Patent Application Laid-Open No. 11-354786) that an ON resistance much lower than that of silicon can be realized by using a semiconductor with a band gap wider than that of silicon, such as silicon carbide (referred to hereinbelow as SiC) or gallium nitride (referred to hereinbelow as GaN), as a semiconductor material of a field effect transistor for power switching (referred to hereinbelow as power MOSFET).
The so-called IGBT is often used as a semiconductor device for power switching, and an inverter is one of applications thereof. FIG. 8 is a circuit diagram illustrating an inverter configuration. As shown in FIG. 8, in a typical three-phase inverter circuit, upper arms 50, 51, 52 and lower arms 53, 54, 55 are connected in series in each of phases U, V, and W and the serially connected structures of the upper and lower arms are connected in parallel. Each arm is constituted by an IGBT and a FWD (diode) that connects a cathode and an anode between the collector and emitter of the IGBT.
In the circuit configuration such as shown in FIG. 8, a load short circuit can occur due to a variety of factors such as erroneous operation caused by an abnormal gate pulse to the IGBT or noise and misconnection. Where the load short circuit occurs, the inverter control system detects an abnormality and an electric circuit flowing to the element is limited or the system is shut down. However, within a short period before the protection circuit is actuated at the time of load short circuit, the IGBT is exposed to a stress state of a high voltage and a large current.
Therefore, a breakdown withstand capability that is called a load short circuit withstand capability is required for the IGBT or FET that is used as a switching element of each arm. The load short circuit capability is an indicator that shows a period in which an element can withstand a stress state of a high voltage and a large current within a short period before the protection circuit is actuated at the time of load short circuit. A standard value for the load short circuit withstand capability is generally such that the element is not broken down within 10 μsec when a gate voltage is applied in the usual ON state under a power source voltage of ⅔ of an absolute rated voltage of the element, but a system has recently been designed such as to shorten further the short circuit detection time in order to realize a design emphasizing the ON voltage.
When an inverter is configured by using a FET using a WBG semiconductor, the FET is highly desirable to have a load short circuit withstand time of the same order as the conventional FET using Si. A mechanism leading to a breakdown during a load short circuit has been analyzed in detail with respect to thin IGBTs that are predominantly used as the switching element (see, for example, M. Otsuki and six more names, “Advanced Thin Wafer IGBTs with New Thermal Management Solution”, Proceedings of ISPSD' 2003, p. 144-147). According to the analysis results, an excessively high generation loss during a load short circuit causes the element temperature to rise. As a result, a leak current of the PN junction rises, thermal burn-up is started, and breakdown occurs.
A band gap of Si is as narrow as about 1.1 eV. Therefore, at a temperature of equal to or higher than 200° C., Si moves locally into an intrinsic region, loses semiconductor properties, and becomes a conductor. Therefore, the element breakdown caused by such a rise in temperature to 200° C. or higher temperature frequently occurs. In order to avoid this breakdown, IGBTs and FETs using Si are designed so that a current at the time of load short circuit assumes an appropriate value, whereby the operation temperature of the semiconductor region is prevented from exceeding a critical point. Alternatively, the load short circuit breakdown is prevented by imparting a current limiting function with an external circuit (see, for example, M. Otsuki and three more names, “The 3rd Generation IGBT Toward a Limitation of IGBT Performance” Proceedings of 5th ISPSD, 1993, p. 24-29).
Si IGBTs employed in inverters are devices using a bipolar effect. Therefore, the saturation current can be limited to a low value, while suppressing the ON voltage. Further, in a high breakdown voltage region of equal to or higher than 600 V, the ON resistance of MOSFET in Si is such that the resistance of MOSFET on the surface is much lower than the resistance of the silicon substrate. Therefore, even if the saturation current of the MOSFET itself is decreased, the effect produced on the ON resistance is small. Thus, a measure designed to lower the ON resistance is not necessarily realized together with a measure designed to increase the load circuit withstand time.